Increased confidence in anthropogenic warming

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After the release of AR5 SPM end of September last year, I was really surprised that the IPCC declared even more confidence in the human influence on climate, despite indications of the contrary. I was puzzled about this contradiction. To my joy I found that in the UK parliamentary Inquiry of Energy and Climate Change Committee into “the science” of UNIPCC AR5 there was some discussion about it. Indeed, when looking at the video I noticed this gem of a question (my own transcript of the video):

[Sir Robert Smith MP]
Is it possible, in the time we got, to summarize why there was an increased confidence in the human influence?

That was the puzzling question that was buggering me for so long. Peter Stott of the Met Office took the word:

[Peter Stott]
We have more evidence across the climate system. And we have evidence in terms of the ocean warming in particular. We understand we have better observations now, we understand not only the changes, but also the variability in ocean temperatures. Of course, if we going to measure temperatures below the surface of the oceans we have to send buoys down to measure those temperatures and so on.

[Sir Robert Smith MP]
Isn’t that more confirming the outcome warming. How does it increase the confidence that the human activity was the cause?

[Peter Stott]
It is providing our, it is increasing our level of conf [interrupted] I am sorry, excuse me,

[Sir Robert Smith MP]
I want to pin down, that is more a symptom of the system warming, but does it increase the confidence that the human activity was the cause?

That was my opinion also. He was talking about warming in general, not about why he thinks there is now more confidence in human influence. Good that Smith noticed it quickly and intervened. Stott continued:

[Peter Stott]
What we are seeking to do is to quantify the overall role of human activity. I mean, our null hypothesis starting point is explaining by internal natural processes and we can rule that out with high confidence levels. Then it comes to the question of the so called attribution question of how we quantify the human role, and that is what we discussed, the starting point could be that it is dominated by natural factors and human factor is rather small. So this is why we have increased our confidence in saying that extremely likely, more than half the observed warming, therefor the dominant cause is human influence. And to do that you need to bring all the evidence to bearing including importantly the ocean temperatures. Previously in AR5 there was a lot of variability, apparently, in the observational record, which we now understood was an artifact of different biases of those measurements. So, that is one example on which we have improved our confidence. But it is not an in terms of ocean temperatures but also in other factors such as the retreating ice and snow. We developed our understanding of the water cycle. We developed an understanding of extremes. When you look into our chapter, chapter 10, you will see we have a table in the back there of 33 rows where we itemized all the evidence, both the evidence, well, we have multiple lines of evidence, and also were we also have remaining uncertainties. And we have a greater wealth of evidence now than we had. With better understanding. With more observations. And also with the improved models as well.

He had a real problem keeping with the question and explained the warming as such. Even after the intervention of Smith the answer stayed really vague and in some places even circular. How disappointing for a lead author and the “Attribution Expert” of the Met Office. What I take from it is “we have more data now”. Indeed, 6 years of data extra. For the ocean temperatures this means 9 years of data in stead of 3. Still not very impressive. This is even less than the standstill of global temperature data and in climate terms insignificant. Not really impressive.

Myles Allen came to the rescue to clarify the matter and came in a real juicy discussion with Peter Lilley and Robert Smith.

[Myles Allen]
Just briefly to come back to the earlier conversation. I recall one of the big arguments in AR4 was over whether human influence via aerosols could be masking the human influences due to greenhouse gases. That was one of the big uncertainties. Now the uncertainties came down, because we are more confident that the aerosol forcing is not at the really high end that was considered possible back in AR4. And that increases our confidence in the size of the total anthropogenic warming over the past 50 years. Because our confidence about anthropogenic cooling [interrupted]

[Peter Lilley MP]
It means it is less?

[Myles Allen]
No, because aerosols cause cooling, because less aerosols and the total anthropogenic warming [interrupted]

[Peter Lilley MP]
There has been less cooling

[Myles Allen]
The total anthropogenic warming … if you are saying you are more confident about the anthropogenic cooling is less, then obviously the total anthropogenic warming is, … then your lower band of total anthropogenic warming goes up. We are more confident that humans are not cooling the climate, therefor we can have greater confidence that the overall effect, including all the warming factors, is towards warming. That’s just one of the factors that play in.

[Peter Lilley MP]
Let’s look at the real world, you know how much warming there has been, it is not how much warming there is in the models. There has been, let’s say 0.7 degrees warming. If that is not the result of 1 degree warming through CO2 and 0.3 degrees cooling through aerosols, which for example we know there is cooling through aerosols, than the amount of warming through CO2 must be quite subtle.

[Myles Allen]
The statement is about the total human induced warming. We have greater confidence in what the total human induced warming is than what we been able to give in AR4, That is an example of an increased confidence, resulting [interrupted]

[Peter Lilley MP]
Increased confidence and a smaller amount.

[Myles Allen]
Actually, no, I don’t believe that is a correct representation of the conclusions. But the key thing is that by ruling out something that might be compensating for the warming effect, by assigning a lower probability that what might be compensating for the the warming effect of CO2, you can increase your confidence that the overall human influence on climate is towards warming.

[Sir Robert Smith MP]
You are talking about the net effect of human activity.

[Myles Allen]
Yes, and that is what the statement is

[Sir Robert Smith MP]
But Mr. Lilley was making the statement that if there is less human input to cooling, then the human input to warming

Very good persistent questions of both MP’s. But they got this nonsensical answer, not even close to a clarification of what they have been asking:

[Myles Allen]
This uncertainty in aerosols is reflected in our uncertainties, specifically the warming attributed due to greenhouse gases. Which is highly relevant to quantifying uncertainty in the future. And as I emphasized before, it is that uncertainty which contributes substantially to the range of uncertainty in future projections. It is this balance between aerosol induced cooling and greenhouse induced warming. But this is specifically focusing on how much warming of the past 50 years is attributed to overall human influence. And that is the situation where our reduced estimates actually help increase our confidence in the total human influence.

Then the chairman interrupted the discussion and put the conversation on the rails of the consensus (well, due to this “higher confidence”). Saved by the bell.

Initially I couldn’t really grasp what he was talking about. But suddenly it dawned on me. It is really simple actually. It took me a while, but I think both Robert Smith and Peter Lilley saw it on the spot and were pushing for the answer.

This is how I think it makes perfectly sense. In the IPCC universe there are two main elements in the temperatures: natural variability and anthropogenic influences. The latter consist of aerosols (which have a cooling effect) and greenhouse gases (which have a warming effect). They assume that the greenhouse effect will be increasing, because our emissions also increase. Now the statement was not about the amount of anthropogenic warming, but about the proportion of warming in the anthropogenic influences.

Big difference.

When being more certain that anthropogenic aerosols have a lower cooling effect, this means that the net anthropogenic forces are now mainly in the warming area. In a way this is indeed an “improvement” over AR4 were the influences of aerosols was less certain. This could explain the change in terminology in AR5 (95% certainty about total human influences) to AR4 (90% certainty about human influences by greenhouse gases alone). This throws another light over the triumphal message that there was more confidence of human attribution in AR5 than in AR4. Did they really wanted to communicate such a meaningless message?

It is not hard to understand that Smith and the other MP’s understood it differently when asking this question. As policy makers they want to know about the certainty of the amount of warming caused by humans, while the scientists were talking about certainty about the proportion of warming within the anthropogenic influences.

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